An ultrasonic sensor having a piezoelectric element mounted on an acoustic matching element is well known. Here, the acoustic matching element improves transmission efficiency of ultrasonic wave, which is transmitted from and received to a device. This ultrasonic sensor includes an ultrasonic wave transmission element for transmitting the ultrasonic wave, and an ultrasonic wave reception element for receiving the ultrasonic wave, which is reflected on an object. Thus, the sensor detects a position of the object disposed around the sensor, a distance between the object and the sensor, a two-dimensional shape of the object or a three-dimensional shape of the object.
The sensor is used for, for example, an echography device used in a medical field. Specifically, JP-H05-123317 teaches an ultrasonic probe having a piezoelectric oscillator and an acoustic matching layer. The piezoelectric oscillator transmits and receives the ultrasonic wave. The piezoelectric oscillator is two-dimensionally stacked under the acoustic matching layer via a common electrode.
When the ultrasonic probe is used for an in-vehicle sensor, it is required for the probe to have environmental resistance so that the probe can be used under vehicle environment. Specifically, moisture or a water drop may adhere to an electrode or a wire connection portion in the oscillator. Thus, the electric performance or structural performance of the probe may be reduced.
Thus, it is required for the ultrasonic sensor to protect from deterioration and failure caused by environmental factor or impact.